DEFINITION OF SEISMOGRAPH

Seismographs are sensitive instruments that

can detect, amplify, and record ground vibrations, especially

earthquakes, producing a seismogram. Numerous seismographs

have been installed in the ground throughout the

world and form a seismograph network, monitoring earthquakes,

explosions, and other ground shaking.

The first very crude seismograph was constructed in

1890. While the seismograph could tell that an earthquake

was occurring, it was unable to actually record the earthquake.

Modern seismographs display Earth movements by

means of an ink-filled stylus on a continuously turning roll of

graph paper. When the ground shakes, the needle wiggles and

leaves a characteristic zigzag line on the paper.

Seismographs are built using a few simple principles. To

measure the shaking of the Earth during a quake, the point of

reference must be free from shaking, ideally on a hovering

platform. To accommodate this need, engineers have

designed an instrument known as an inertial seismograph.

These make use of the principle of inertia, which is the resistance

of a large mass to sudden movement. When a heavy

weight is hung from a string or thin spring, the string can be

shaken and the big heavy weight will remain stationary.

Using an inertial seismograph, the ink-filled stylus is attached

to the heavy weight and remains stationary during an earthquake.

The continuously turning graph paper is attached to

the ground, and moves back and forth during the quake,

resulting in the zigzag trace of the record of the earthquake

motion on the graph paper.

Seismographs are used in series; some set up as pendulums

and some others as springs, to measure ground motion

in many directions. Engineers have made seismographs that

can record motions as small as one hundred-millionth of an

inch, about equivalent to being able to detect the ground

motion caused by a car driving by several blocks away. The

ground motions recorded by seismographs are very distinctive,

and geologists who study them have methods of distinguishing

between earthquakes produced along faults,

earthquake swarms associated with magma moving into volcanoes,

and even between explosions from different types of

construction and nuclear blasts. Interpreting seismograph

traces has therefore become an important aspect of nuclear

test ban treaty verification.

In the late 19th century E. Wiechert introduced a seismograph

with a large, damped pendulum used as the sensor,

with the damping reducing the magnitude of the pendulum’s

oscillations. This early seismograph recorded horizontal

motions and used a photographic recording device. Wiechert

soon introduced a new seismograph with a mechanical

recording device, with an inverted pendulum that could

vibrate in all horizontal directions. The pendulum was supported

by springs that helped stabilize the oscillations and

furthered the productivity of the seismograph. Wiechert’s

assistant, named Schluter, introduced a vertical recording

device. He moved the mass horizontally away from the axis

of rotation and maintained it there with a vertical spring. In

doing so he was able to record vertical displacement, which

helped record many of the complex movements associated

with earthquakes.

In the 20th century seismographs that recorded movements

using a pen on a rotating paper-covered drum were

introduced, with alternative devices including those that

recorded movements using a light spot on photographic film.

More sophisticated seismographs that are able to record

movements in three directions (up-down, north-south, and

east-west) were introduced, and electronic recording of relative

motions became common.

See also EARTHQUAKES; SEISMOLOGY.

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